This invention relates to an improved method for cooling the hot product gas obtained when coal is partially combusted in a conventional slagging coal gasifier. More particularly, this invention is directed to a process for direct quenching of the hot product gas of a slagging coal gasifier in a tubular quench zone whereby deposition of sticky, molten slag particles, typically dispersed in such product gases, on the quench zone walls is minimized or avoided during the period before the slag particles become sufficiently cooled to lose their stickiness.
The partial combustion or gasification of solid carbon-containing fuels such as coal to produce gases having valve as residential and industrial fuels, as starting materials for synthesis of chemicals and fuels and as an energy source for generations of electricity has long been recognized and practiced on varying scales throughout the world. In the case of coal gasification, a number of different gasification processes have been developed to take into account factors such as the coal source employed, the gasifying medium used and the use sought to be made of the product gas. While these processes may be classified in a variety of ways, they generally fall into two distinct groups with respect to the condition in which the non-carbonaceous, mineral residue is removed from the gasification zone, i.e., dry ash in a nonslagging operation or slag in a slagging operation. These two different process groupings derive primarily from the temperatures employed in the gasification zone itself -- i.e., the nonslagging gasifiers are operated at reaction temperatures, usually less than 1400.degree. C, below those at which the contained ash will fuse while the temperatures employed in slagging gasifiers are sufficient, usually 1500-2700.degree. C, to convert the dry ash into a molten slag. Though advantages exist for gasification processes falling into each group, the processes employing slagging coal gasifiers are generally considered to be the most flexible at least in terms of the variety of coal feedstocks which can be suitably employed. That is, operation of coal gasifiers under nonslagging conditions is generally limited to weakly coking coals of low ash content because of the difficulty in removing ash with grates and other mechanical devices whereas in operation at slagging conditions, almost any coal can be suitably employed since the ash becomes a free-flowing fluid under slagging conditions and, as a result, is quite simply and easily removed from the gasifier. A good general review of a variety of coal gasification processes appears in the Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 10, pp. 353-388, Interscience (1966).
One process employing a slagging coal gasifier which has had rather wide application is the Koppers-Totzek process. This process which is described in an article by F. Totzek in "Brennstoff-Chemie," Vol. 34, pp. 361-367 (1953), has the capability of handling just about any coal including lignites with up to 30% ash or mineral contents. While a significant portion of the molten slag is removed at the bottom of the gasifier, the product gas of this process like other processes employing slagging gasifiers still contains a significant quantity of mineral matter in the form of a suspension or mist of molten or partly molten particles.
Primarily because of the impure nature of the mineral matter in typical coals, being mixtures of silica and various metal oxides, the molten or partly molten slag will not have a specific melting point but rather will solidify over a melting range which may cover many hundreds of degrees. Thus, since it is usually necessary to cool the coal gasifier effluent prior to further processing, the molten or partly molten slag contained therein is or can become sticky, at least temporarily, on cooling. In a typical application, the gas leaving the reactor has a temperature, as a rule higher than 1400.degree. C, at which the ash is quite fluid. For further processing, this crude product gas has to be cooled down to a temperature, for example 300.degree. C, through a rather broad range of temperatures at which the slag is sticky, i.e, slag from coal usually being sticky in the temperature range of 1500 -900.degree. C. When the slag particles are no longer sticky, they can be easily removed by known techniques such as cyclones, bind separators, filters or similar devices. However, in the transition between being highly fluid molten liquid and solid nonsticky particles, these slag particles exhibit sufficient stickiness that they can cause extreme difficulties in processing by adhering to and forming deposits on walls, valves, outlets, etc., of process equipment immediately downstream of the gasifier. These deposits tend to build up and as a result interfere with good operation of the process and even lead to complete blocking. Accordingly, the instant invention provides a process for cooling down the product gas of a slagging coal gasifier in which the harmful effects of the stickiness of molten slag particles contained therein is minimized and even completely eliminated.